Precoding with reduced feedback for coordinated multipoint transmission on the downlink

ABSTRACT

Techniques for reducing the number of bits needed to specify the best precoding vector for each mobile station in a wireless communication network that employs multi-point transmission are disclosed. An exemplary method begins with the estimation of path loss between a mobile station and each of a plurality of geographically separated transmitter sites, each transmitter site having at least one transmitter antenna. Based on the estimated path losses, one of a plurality of pre-determined subsets (codebooks) of a pre-determined set of antenna precoding vectors is selected. A group index identifying the selected subset is then transmitted to the mobile station. Subsequently, a vector index is received from the mobile station, the vector index corresponding to a precoding vector in the selected subset, and data is transmitted to the mobile station, using the precoding vector applied to the transmitter antennas at the plurality of transmitter sites.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/121,775, filed 11 Dec. 2008by Kambiz Zangi, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates generally to wireless communicationssystems, and more particularly relates to methods and apparatus fordetermining transmission parameters in a mobile communications networkusing coordinated transmissions from multiple transmitter sites.

BACKGROUND

Precoding of multi-antenna transmissions is an increasingly populartechnique used in several advanced wireless communications standards.Precoding techniques include single-layer beamforming, where the samesignal is emitted from each of several transmit antennas, but withdifferent precoding weights applied to each of the antennas so that thesignal power is maximized at the receiver output. When the receiver hasmultiple antennas, precoding is used for multi-layer beamforming inorder to maximize the throughput performance of a multiple receiveantenna system. With multi-layer precoding, multiple data streams aresimultaneously transmitted, with independent weights applied to eachantenna to maximize the link capacity or quality.

The Wideband Code-Division Multiple Access (W-CDMA) and Long-TermEvolution (LTE) standards promulgated by the 3rd-Generation PartnershipProject (3GPP) each permit linear precoding on the downlink, whenmultiple transmit antennas are used to serve a mobile station (UserEquipment, or “UE”, in 3GPP parlance). The transmitter antenna precodingvector that maximizes the data rate received by each mobile depends onthe instantaneous downlink channel (including fading) to the mobile;hence, implementing this optimal precoder requires the network toacquire channel state information characterizing the propagationconditions between each of the transmit antennas and the mobile station.In an Orthogonal Frequency-Division Multiple Access (OFDMA) system likeLTE, this channel state information must be acquired for each pair ofreceive/transmit antennas (i.e., M×N single-input/single-outputchannels, where M is the number of transmit antennas and N is the numberof receive antennas).

Codebook-based linear precoding is a technique that reduces the amountof information needed to feedback from the mobile to the network forimplementing the linear precoding. With a traditional codebook-basedprecoding, a fixed set of allowed precoder weighting vectors (acodebook) is chosen a priori, and each precoding vector in this set isassigned a unique index that is known to both the network and the mobilestations. Each mobile station measures its downlink channel (of sizeM×N) and determines the “best” precoding vector belonging to thecodebook, given the downlink channel measurements. The mobile stationfeeds back an index of this best precoding vector to the network, sothat subsequent transmissions from the network can be precoded accordingto the selected vector.

Assuming a codebook consisting of L precoding vectors, log₂(L) bits areneeded to uniquely identify a single precoding vector. Typically,log₂(L) bits is fewer than the number of bits needed to characterize theM×N downlink channel each mobile station sees; thus, the codebook-basedapproach reduces the amount of signaling information that must betransmitted between the mobile station and the network.

In WCDMA and LTE systems, a multi-antenna transmission to a mobilestation is generally transmitted from only a single point, i.e., asingle transmitter site. In other words, the M transmit antennas used toserve a given mobile are usually co-located. In this case, the channelbetween every one of these antennas and the mobile station has the samepath loss and shadowing. The codebooks in LTE and WCDMA are designedspecifically for the case when all the transmit antennas are co-located,i.e., for the case in which the path loss between each of thetransmitter antennas and the mobile station is the same, or very closeto the same. (The term “path loss” is sometimes used to refer only tothe propagation loss caused by distance between the transmitter andreceiver, whether a free-space model, i.e., where path loss isproportional to the distance squared, or an empirically derived model,e.g., where path loss is proportional to the distance raised to thefourth power, is used. “Shadowing,” on the other hand, generally refersto losses caused by particular features of the environment in a givenscenario, such as losses caused by the proximity of a large building orgeological feature. In the remainder of this disclosure, however, theterm “path loss” is generally intended to include both of thesephenomena, unless the context indicates otherwise, but to exclude thedistinct phenomena of fading, which results from the destructivecombining of multipath components of the transmitted signal at thereceiver.)

In developing the specifications for the so-called LTE-Advanced system,3GPP members are considering the use of true multi-point transmission(i.e., from multiple transmitter sites), where the M transmit antennasused to serve a given mobile station could be located at severaldifferent geographical locations. In these scenarios, the channelsbetween the mobile station and various ones of these antennas could havedifferent path losses. Conventional codebook-based techniques forspecifying a particular precoding vector to be used, given rapidlyvarying channel conditions, are poorly suited for the multi-pointtransmission scenario.

SUMMARY

With multi-point transmission, mobile stations are subject to differentcombinations of path losses between the mobile stations and each of theseveral transmitter sites, depending on the mobile stations' locations.Hence, the total number of possible codebook entries needed to implementcodebook-based precoding can become quite large with multipointtransmission. Conventional approaches in which a single precoding vectoris identified, from all possible precoding vectors, would require alarge number of bits, and thus a large signaling load between the mobilestation and the network. This large signaling load consumes systemresources that could otherwise be used for transmission of data to theusers in the system.

Embodiments of the present invention include techniques for reducing thenumber of bits needed to specify the best precoding vector for eachmobile station in a wireless communication network that employsmulti-point transmission. Because the path losses (including shadowingeffects) between a given mobile station and the M transmit antennas varyat a much slower rate than the fading due to multipath effects, theoptimal choice of a codebook adapted to a particular combination of pathlosses changes at a rate much slower than fading. Thus, variousembodiments of the invention involve specifying any one precoding vectoras a two step process. First, a codebook in which a particular desiredprecoding vector resides is specified. This specification can be donewith log₂(Q) bits, given Q possible codebooks. Each codebookconceptually corresponds to a particular combination of path lossesbetween a mobile station and the M transmit antennas to be used formulti-point transmission to the mobile station. Second, an index to theparticular desired precoding vector within the earlier identifiedcodebook is specified. This specification can be done with log₂(L) bits,given that each codebook includes no more than L precoding vectors.

At a slow rate (e.g., once every few seconds, or even less frequently),each mobile station can update its choice of codebook, given the recentpath loss conditions. Once data transmission to a mobile station isbegun, the mobile station repeatedly identifies the best precodingvector in the selected codebook, based on current fading conditions.Thus, the particular precoding vector used for a given transmission isupdated more frequently than the codebook from which the precodingvectors are selected. As long as any updates to the codebook are muchless frequent than the updates to the particular precoding vector used,the overall amount of feedback bits required to specify the bestprecoder with multi-point transmission is very similar to the amount offeedback bits needed with single-point transmission with a singlecodebook of size L.

Accordingly, one exemplary method for determining transmissionparameters in a mobile communications network using coordinatedtransmissions from multiple transmitter sites begins with the estimationof path loss between a mobile station and each of a plurality ofgeographically separated transmitter sites, each transmitter site havingat least one transmitter antenna. Based on the estimated path losses,one of a plurality of pre-determined subsets (codebooks) of apre-determined set of antenna precoding vectors is selected. A groupindex is then transmitted to the mobile station, the group indexidentifying the selected subset of antenna precoding vectors.

Subsequently, a first vector index is received from the mobile station,the first vector index corresponding to a first antenna precoding vectorin the selected subset, and first data is transmitted to the mobilestation, using the first antenna precoding vector applied to thetransmitter antennas at the plurality of transmitter sites.

In some embodiments of the above-summarized method, estimating path lossbetween a mobile station and each of the plurality of geographicallyseparated transmitter sites comprises receiving channel state reportsfrom the mobile station and estimating the path losses based on thechannel state reports, wherein the channel state reports characterizedownlink path loss, received downlink signal quality, or both. In otherembodiments, estimating path loss between a mobile station and each ofthe plurality of geographically separated transmitter sites may insteadcomprise estimating uplink path losses, based on measurements of uplinksignals from the mobile station, and then estimating correspondingdownlink path losses based on the estimated uplink path losses.

In some embodiments, site information is transmitted to the mobilestation prior to receiving the first vector index from the mobilestation, the site information identifying the plurality ofgeographically separated transmitter sites to the mobile station.

As suggested above, the “best” precoding vector within a selectedcodebook may change over time, due to changing fading conditions. Thus,in some embodiments, any of the methods discussed above may furthercomprise receiving a second vector index from the mobile station, thesecond vector index corresponding to a second antenna precoding vectorin the selected subset, and transmitting second data to the mobilestation, using the second antenna precoding vector applied to thetransmitter antennas at the plurality of geographically separatedtransmitter sites.

The above-summarized methods may, in some embodiments, be suitable forimplementation in or at one or more fixed nodes of a wirelesscommunication system using coordinated transmissions from multipletransmitter sites. Thus, various arrangements of processing circuits inone or more fixed nodes of such a mobile communications network aredisclosed herein, these arrangements generally corresponding to themethods summarized above.

Other exemplary methods might be more suitable for implementation in amobile station operating in a mobile communications network usingcoordinated transmissions from multiple transmitter sites. In some suchmethods, transmitter site information identifying a plurality ofgeographically separated transmitter sites is received, and a groupindex identifying a selected one of a plurality of pre-determinedsubsets of a pre-determined set of antenna precoding vectors isdetermined. In some embodiments, the group index is determined by simplyreceiving the group index from the mobile communications network; inothers, determining the group index comprises estimating path lossbetween the mobile station and each of the plurality of geographicallyseparated transmitter sites, selecting one of a plurality ofpre-determined subsets of a pre-determined set of antenna precodingvectors, based on the estimated path losses, and identifying the groupindex corresponding to the selected subset. In these latter embodiments,the group index may be transmitted to the mobile communications network.

In either case, channel conditions between the transmitter antennas andthe mobile station are evaluated, and a first antenna precoding vectorfrom the selected subset is selected, based on the channel conditions. Afirst vector index is transmitted to the mobile communications network,the first vector index identifying the first antenna precoding vector,given the selected subset. Subsequent transmissions from the network,using the identified precoding vector may be processed at the mobilestation using the first antenna precoding vector, in some embodiments.

Again, the “best” precoding vector within a given codebook may change,as fading conditions change. Thus, several methods according to thepresent invention comprise re-evaluating channel conditions between thetransmitter antennas and the mobile station, selecting a second antennaprecoding vector from the selected subset, based on the re-evaluatedchannel conditions, and transmitting a second vector index to the mobilecommunications network, the second vector index identifying the secondantenna precoding vector, given the selected subset.

A mobile station containing processing circuits configured to carry outone or more of the mobile station-related methods summarized above isalso described herein. Of course, the present invention may be carriedout in other ways than those specifically set forth herein withoutdeparting from essential characteristics of the invention. Indeed, uponreading the following description and viewing the attached drawings, theskilled practitioner will recognize that the described embodiments areillustrative and not restrictive, and that all changes coming within themeaning and equivalency range of the appended claims are intended to beembraced therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary wireless communication network employingmulti-point transmission.

FIG. 2 is a schematic diagram illustrating additional details of amulti-point transmission system according to some embodiments of thepresent invention.

FIG. 3 is a process flow diagram illustrating an exemplary method fordetermining transmission parameters in a mobile communications networkusing coordinated transmissions from multiple transmitter sites.

FIG. 4 is a process flow diagram illustrating an exemplary method in amobile station for determining transmission parameters for use incoordinated multi-point transmissions.

FIG. 5 is a schematic diagram illustrating features of an exemplarymobile station.

DETAILED DESCRIPTION

With multi-point transmission, the best codebook (set of precodingvectors) for a given mobile station depends on the M-tuple ofpath-loss/shadowing between each of the M possible transmit antennas andthe mobile station. If it is assumed that Q distinct combinations ofpath-loss between a mobile station and the M possible transmit antennasare considered, the most straight-forward way of specifying the bestprecoder for each sub-carrier would require log₂(Q*L)=log₂(Q)+log₂(L)bits, assuming that each of Q codebooks includes L precoding vectors.Since there are many possible combination of path loss combinationscorresponding to the possible combinations of transmitter sites andmobile station positions in a given network, Q could be quite large in atypical cellular system. As a result, the amount of feedback forimplementing codebook-based precoding in a multi-point transmissionsystem could be much larger than the amount of feedback required in asingle-point system.

If it is assumed that each mobile station can be served by up to Stransmitter sites, and that each site contains P transmit antenna ports,then the maximum number of antennas serving each mobile station will beM=P*S. Of course, the number of transmitter sites actually used at agiven time may vary, depending on network configuration, the position ofthe mobile station within the network, and various other factors such asthroughput demands from the mobile station, loading of the network, andso on.

FIG. 1 illustrates an example of a multi-point network scenario in whichmobile station 110 is receiving coordinated multi-point transmissionsfrom three base stations 120, which are tied to a public data network(not shown) via access gateway 130. Each of the base stations 120 istransmitting signals to mobile station 110 using up to three transmitterantennas 140. However, those skilled in the art will appreciate thateach base station in a coordinated multi-point transmission network neednot use all available antennas for a given transmission, and that somebase stations may have more or less available antennas than others. Inthe pictured system, therefore, from three to nine antennas 140 may beused for a given transmission, assuming that all three of the picturedbase stations 120 participate.

Because the antennas 140 at any given transmitter site 120 are close toone another, the path losses between mobile station 110 and the antennasat a given base station 120 are effectively the same. However, the pathlosses between mobile station 110 and each of the different basestations 120 may vary widely. As a first example, assume that the pathlosses between mobile station 110 and each of base stations 120-A and120-B are roughly equal, but that the path loss between mobile station110 and base station 120-C is higher, e.g., 10 dB greater. If mobilestation 110 is served only by base stations 120-A and 120-B, and onlyone antenna 140 is used per base station, then each codebook entry is atwo-element vector (one element for each transmit antenna 140), and theparticular precoding vector to be used at a given time is selected froma codebook that is optimized for a scenario in which the path losses arealmost equal. If mobile station 110 is instead served only by basestations 120-B and 120-C, then the optimal precoding vector for a givenset of instantaneous channel conditions is selected from a codebookoptimized for a path loss scenario where one path loss is approximately10 dB greater than the other.

As another example, if all three antennas 140 at each of the basestations 120 are used, then each codebook entry (precoding vector) willinclude nine elements. In this case, three sites are used, so thegrouping of the allowed precoding vectors into codebooks is based on athree-part combination of path losses from the mobile station 110 to thebase stations.

Thus, a pre-determined set of precoding vectors may be divided into aplurality of pre-determined subsets, with each subset (codebook)corresponding to one or more path loss scenarios. The pre-determined setof precoding vectors and the pre-determined subsets may be specified ina particular standard, so that it is known to the network and to mobilestations operating in the network.

Those skilled in the art will appreciate that the number of possiblecombinations of path losses can be very large. However, ranges of pathloss scenarios can be mapped to codebooks that are optimal ornear-optimal for at least a part of that range. In this way, the numberof different codebooks can be kept at a manageable level.

Thus, in various embodiments of the invention, a number of codebooks aredefined, where each codebook includes several precoding vectors. Inother words, a pre-determined set of precoding vectors may be dividedinto a plurality of pre-determined subsets, with each subset (codebook)corresponding to one or more path loss scenarios. Given Q codebooks,L(q) may be used to indicate the number of precoding vectors in the q-thcodebook, which is designated B(q). Furthermore, a particular precodingvector may be readily identified by a combination of two indices: agroup index, specifying one of the Q codebooks, and a vector index,specifying one of the L(q) precoding vectors in the codebook identifiedby the group index. A particular pre-determined set of precoding vectorsand the pre-determined subsets may be specified in a particularstandard, so that it is known to the network and to mobile stationsoperating in the network. With this shared knowledge, a mobile stationand the network can communicate about the precoding vectors using onlythe group and vector indices.

As noted above, because the antennas at a given transmitter site arelocated very close to one another, as compared to the distance to agiven mobile station, the path loss (including shadowing) from all ofthe antennas at a given site to the given mobile station will be almostthe same. The path loss corresponding to a given transmitter site may bedenoted by G(s). Then, the path losses from all the M transmit antennasto the mobile station are specified by a vector G (of length S) definedas:G=[G(1), G(2), . . . , G(s)]  (1)

Periodically, a network configured according to some embodiments of thepresent invention forms an estimate of G, denoted by Estimated_G, basedon uplink measurements or based on reports from the UE, for each mobilestation. Note that this estimate process includes (or presupposes)choosing the specific S sites that will be serving a given mobilestation on the downlink. This selection, as well as the estimation ofthe path losses, may be performed on a relatively infrequent basis,e.g., at time intervals many times the expected fast-fading time. Thus,the estimation process may comprise the averaging of several uplinkmeasurements or channel reports from the mobile station, so that fadingeffects are averaged out.

There will be a mapping from the estimated values of G to the bestcodebook for the mobile station. This mapping may be denoted asMapping(G); the output of Mapping(G) is one of the Q codebooks definedpreviously (or an index to one of the Q codebooks defined previously).

Once the network determines the best codebook for a given mobilestation, the network signals to the mobile station, e.g., on a downlinkcontrol channel, its selection of the best codebook for this mobilestation. Because the mobile station already “knows” the contents of eachof the pre-determined codebooks, only the index of the chosen codebookneeds to be signaled; the elements of each of the vectors in thecodebook need not be signaled. Those skilled in the art will appreciatethat the network must also signal to the mobile station the sites fromwhich the mobile station will be receiving its transmission; thissignaling may be sent in a single message, along with the codebookindex, or in a separate message.

After receiving an indication of the chosen codebook from the network,as well as the set of transmitter sites that will be serving it, themobile station then reports to the network a vector index associatedwith its choice of precoding vector within the chosen codebook. Thisvector index is selected by the mobile station based on evaluating“instantaneous” (short-term) channel conditions between the M transmitantennas and the mobile station. These channel conditions changequickly; thus, the channel conditions are re-evaluated and the selectionof the vector index is updated on a frequent basis (i.e., at several ormany times the rate at which the codebook is updated).

Next, the network will use the vector index reported by the mobilestation, as well as the network's knowledge of the codebook it assignedto the mobile station, to identify the particular precoding vector thatshould be used for transmitting to the mobile station on the downlinkfrom the S sites that the network has chosen for serving this mobilestation, given the previously identified codebook.

If dedicated reference signals are used for transmitting to the mobilestations, the network will not need to signal to the UE which precoderis actually used for each transmission to the mobile station. However,some networks may be configured so that the network is permitted toselect a pre-coding vector other than the one selected and identified bythe mobile station. In this case, if no dedicated reference signal isused, then the network must signal to the mobile station which precodingvector (within the codebook that was previously chosen for the mobilestation) was used for each transmission to the mobile station, so thatthe mobile station can properly decode the common reference signals(which are not weighted by UE-specific precoding weights).

FIG. 2 is a schematic diagram illustrating functional components of thefixed network portion of a wireless communication system, according tosome embodiments of the invention. In the pictured arrangement, two datastreams (designated X₁ and X₂) are targeted to a particular mobilestation (not shown). Each data stream is separately encoded andmodulated, using encoding/modulation circuits 210-1 and 210-2. Vectorweights, supplied by processing circuits 250, are applied to each of themodulated data streams at weighting circuits 220-1 and 220-2. Asdescribed in further detail below, the vector weights are precodingvectors selected and identified according to the inventive techniquesdescribed herein. The output of the weighting circuits 220, i.e., theweighted transmitter symbols are applied to the antennas 240 at each ofS sites. In the illustrated system, two data streams are spatiallymultiplexed, thus each antenna is provided weighted signalscorresponding to streams X₁ and X₂; these signals are summed for eachantenna using adders 230. Of course, in other systems, or at other times(e.g., depending on channel conditions) only a single stream (or morethan two streams) might be used.

The precoding vectors applied to the modulated data streams at weightingcircuits 220-1 and 220-2 are provided by processing circuits 250, whichcomprise a vector selection circuit 252 and a controller circuit 254,which comprise one or more microprocessors, microcontrollers, or thelike, configured with appropriate program code stored in program memory256. The processing circuits 250 have access to a vector database 260,which may stored in one or more memory devices, including, in someembodiments, the same memory device (or devices) used for program memory256. Those skilled in the art will readily appreciate that, in someembodiments, vector selection circuit 252 and controller circuit 254 maybe implemented using a single, shared, microprocessor, while in otherembodiments the functions performed by the illustrated circuits may bespread among two or more circuits, in one or more nodes of the wirelesscommunication system. Those skilled in the art will further appreciatethat the advantages and disadvantages of centralizing or distributingthe functionality of vector selection circuit 252 and controller circuit254 will vary, depending on the underlying architecture of the wirelesscommunication system.

FIG. 3 illustrates an exemplary method for determining downlinktransmission parameters in a mobile communication network employingmulti-point transmissions. The method illustrated in FIG. 3, andvariations thereof, may be implemented with the processing circuits 250of FIG. 2, or similar circuit arrangements.

In any case, the method illustrated in FIG. 3 begins with the estimationof path losses between a mobile station and each of a plurality oftransmitter sites, as shown at block 310. In some embodiments, the pathlosses are estimated at each of several base stations that are currentlytransmitting signals to the mobile station or that are expected totransmit signals to the mobile station in the future; in others, one ofthe base stations (e.g., an “anchor” base station) or anothercentralized node in the network estimates the path losses based on datareported to it from the various base stations. In some embodiments, thepath losses are estimated based on channel quality data characterizingdownlink signal quality or downlink path loss (e.g., signal strength,signal-to-noise ratios, or the like) and sent to the network by themobile station; this data may be sent to each of several base stationsor to a single base station. In still other embodiments, the estimationof path losses may comprise first estimating uplink path losses, basedon measurements of uplink signals from the mobile station, and thenestimating corresponding downlink path losses based on the estimateduplink path losses.

The path losses reflect the configuration of the base stations and themobile station in a macro sense, in that the path losses reflectpropagation loss as a function of the distance between each base stationand the mobile, as well as shadowing caused by physical features of theenvironment (hills, valleys, buildings, etc.). These path losses, whichdo not include fading effects resulting from phase-sensitivecombinations of multipath signals, change relatively slowly. Thus, insome embodiments, the path loss estimates are based on averages ofseveral measurements, taken over a period of time that exceeds theexpected fading intervals.

Given the estimated path losses, a best one of a plurality of codebooks(i.e., a subset of a pre-determined set of antenna precoding vectors,each subset including a plurality of antenna precoding vectors) isselected. As discussed above, each of the plurality of codebookscorresponds to a particular combination of path loss conditions, given aparticular number of transmitting sites and antennas. Thus, the codebookcorresponding to path loss conditions that best match the estimated pathlosses is selected, as shown at block 320. Because the mobile station isprogrammed with knowledge of the predetermined codebooks, the selectedcodebook can be identified to the mobile station by simply transmittinga group index to the mobile station, as shown at block 330, with thegroup index uniquely identifying the selected codebook.

As discussed above, the number of bits needed to specify the group indexdepends on the number of codebooks, Q. If there are 32 codebooks, forexample, then five bits (log₂ 32) are needed to specify the group index.The group index is transmitted relatively infrequently, however, as thepath loss conditions are generally expected to change slowly; thus, evenlarger numbers of codebooks are feasible.

Given a selected codebook, the mobile station estimates channelconditions between the base stations and the mobile, including fadingeffects, and selects a best antenna precoding vector from the selectedcodebook. The mobile station can then identify the selected antennaprecoding vector to the network using a vector index that corresponds tothe selected precoding vector within the selected codebook. Because thevector index only needs to identify a particular vector within analready determined codebook, the vector index can be relatively short,depending only on the maximum number of precoding vectors in any givencodebook. Thus, for example, four bits (log₂ 16) are sufficient toidentify a particular vector in any given codebook if no codebookincludes more than sixteen vectors.

The vector index is received from the mobile station, as shown at block340, and is used to retrieve the identified precoding vector from thepreviously identified codebook. One or more data streams are thenweighted, using the selected precoding vector, and transmitted to themobile station, as shown at block 350. Of course, some systems may beconfigured so that the vector index received from the mobile stationindicates only the mobile station's preference; these systems may beconfigured to occasionally over-ride the indicated preference, based onthe network's knowledge of channel conditions, network loading, etc. Inthese systems, if a precoding vector other than the one preferred by themobile station is used, then the network may need to signal the mobilestation to indicate the precoding vector that is actually used. Assuggested above, this is particularly true if common reference signals(i.e., downlink reference signals for multiple mobile stations) areused, as each mobile station then needs to know that the commonreference signals are weighted differently than data signals.

The “instantaneous” channel conditions at the mobile station changerelatively rapidly, compared to the path losses, due to fading effects.Thus, for any given path loss scenario, the best precoding vector withina selected codebook is likely to vary from one moment to the next. Thus,in some embodiments of the invention, the mobile station is configuredto repeatedly evaluate channel conditions, to update its preferredprecoding vector as needed, based on the re-evaluated channelconditions, and to transmit new vector indices, as needed, to thenetwork. At the same time, the network periodically updates itsestimates of the relevant path losses, and selects a new “best” codebookas needed. These processes are illustrated in FIG. 3. As shown at block360, for instance, the network periodically checks to see whether a newvector index has been received before transmitting a second burst ofdata to the mobile station. If so, a subsequent data transmission usesthe newly selected vector index. The network also periodically checks tosee whether the path losses have changed sufficiently to warrant theselection of a new best codebook, as shown at block 370. If so, a newcodebook is selected, and a new group index identifying the selectedcodebook is sent to the mobile station, as shown at blocks 320 and 330.

Of course, because the expected rates of change for the group index andthe specific vector index differ, the rate at which these updates areperformed will also vary. In fact, vector index changes are likely tooccur many times more often than group index changes, under many networkconfigurations and scenarios. Thus, the signaling described above islikely to be dominated by the vector index signaling. Because the numberof bits needed to signal the vector index is relatively small (e.g., 4bits, compared to the 9 bits that would be needed to uniquely identifyone of 16×32 ungrouped precoding vectors), the signaling overhead isreduced.

While FIG. 3 illustrated an exemplary process for selecting andidentifying antenna precoding vectors at the network. FIG. 4 illustratesa corresponding process that can be implemented in a mobile stationoperating in a mobile communications network using coordinatedtransmissions from multiple transmitter sites. As shown at block 410,the illustrated method begins with the receipt of transmitter siteinformation identifying two or more transmitter sites; one or several ofthese transmitter sites may have two or more antennas for downlinktransmission. This information identifies the downlink signals that themobile station will evaluate to select preferred antenna precodingvectors for downlink transmissions.

As shown at block 420, the mobile station receives a group index fromthe network, the group index identifying a selected one of a pluralityof pre-determined subsets (i.e., codebooks) of a pre-determined set ofprecoding vectors. As discussed earlier, the network and the mobilestation share knowledge of the subsets of precoding vectors; thesesubsets may be specified in an industry standard, in some embodiments,specific to a particular network in others, or even developed in an adhoc fashion in still others. Of course, in the latter cases, a mechanismfor providing the mobile station with shared knowledge of the codebookgroupings is needed, such as pre-programming and/or broadcast signaling.

As shown at block 430, the mobile station evaluates the downlink channelconditions between the mobile station and each of the identifiedtransmitter sites. These evaluations are performed over a short timeinterval, so that the evaluations capture the short term channelconditions, rather than the longer-term, more stable, path lossconditions. Based on the evaluated channel conditions, a preferredantenna precoding vector is chosen from the codebook identified by thegroup index, as shown at block 440. A vector index identifying the choseprecoding vector, given the selected group, is then transmitted to thenetwork, as shown at block 450. As discussed above, this vector indexcan be much shorter than would be needed to uniquely identify aprecoding vector out of an ungrouped set of all available precodingvectors.

As with the network-based method illustrated in FIG. 3, the selection ofa preferred vector index may be updated periodically. In addition, a newgroup index may be signaled to the mobile station from time to time.Thus, as shown at blocks 460 and 470, the mobile station periodicallychecks whether a new group index has been received, specifying a newcodebook, or whether new site info has been received, specifying a newset of transmitter sites. In any case, the short-term channel conditionsare periodically re-evaluated, and a new preferred precoding vectorchosen, as needed.

Those skilled in the art will appreciate that variations of theabove-described techniques are possible. For instance, the process flowdiagrams of FIGS. 3 and 4 illustrate a process in which the bestcodebook to use at any given time is selected at one or more nodes onthe fixed side of the communications network. In other embodiments ofthe invention, the codebook is instead selected at the mobile station.Thus, for example, the “receiving” step illustrated at block 420, inwhich a group index is determined by a mobile station by simplyreceiving it from the network, may be replaced, in some embodiments,with a step in which a group index identifying a particular subset ofpre-coding vectors is determined at the mobile station itself. In someof these embodiments, the mobile station determines the group index byestimating path losses between the mobile station and each of severalbase stations, selecting one of a plurality of pre-defined codebooksbased on the estimated path losses, and identifying the group index thatcorresponds to the selected codebook. In these embodiments, the mobilestation may be further configured to transmit the group index to themobile communications network.

Those skilled in the art will further appreciate that theabove-described mobile station-based methods may be readily adapted tomobile stations of various types and having various systemarchitectures. An example of such a mobile station 500 is illustrated inFIG. 5, and includes a receiver front-end circuit 510, a transmittercircuit 520, and baseband processing circuitry 530. In the picturedembodiment, baseband processing circuitry 530 includes one or moremicroprocessors, microcontrollers, digital signal processors, and thelike, each or any of which may be configured with appropriate software,including program instructions for carrying out one or more of thetechniques discussed above, stored in program memory 535. Mobile station500 further includes a vector database 540, which includes all of theavailable precoding vectors arranged in a format so that a particularvector can be identified by a group index, identifying a group ofvectors, and a vector index, identifying a particular vector within agiven group. Vector database 540 may be stored in any of a variety ofknown memory types and/or configurations, and may be stored in the samememory used for program memory 535, in some embodiments.

Of course, the illustrated mobile station 500 is provided only as anexample; those skilled in the art will appreciate that various mobilestation types and configurations can be adapted according to thetechniques described herein. Likewise, the inventive techniquesdescribed herein can be applied to mobile communications networksoperating according to any of a variety of mobile communicationsstandards, including, but not limited to, LTE and W-CDMA standardspromulgated by the 3GPP. Indeed, these techniques may be adapted tocommunications systems that have not been developed. Thus, those skilledin the art will appreciate that the present invention may be carried outin other ways than those specifically set forth herein without departingfrom essential characteristics of the invention, and the presentembodiments are therefore to be considered in all respects asillustrative and not restrictive. All changes to the specificallydescribed embodiments that come within the meaning and equivalency rangeof the appended claims are intended to be embraced therein.

1. An arrangement of processing circuits in one or more fixed nodes of amobile communications network using coordinated transmissions frommultiple transmitter sites, the arrangement of processing circuitscomprising: one or more memory circuits storing a set of antennaprecoding vectors, wherein each antenna precoding vector is indexed by agroup index and by a vector index; one or more precoding selectorcircuits configured to estimate path loss between a mobile station andeach of a plurality of geographically separated transmitter sites, eachtransmitter site having at least one transmitter antenna; and to selectone of a plurality of pre-determined subsets of the set of antennaprecoding vectors, based on the estimated path losses; and one or morecontroller circuits configured to transmit a first group index to themobile station via one or more of the geographically separatedtransmitter sites, the first group index identifying the selected subsetof antenna precoding vectors; to receive a first vector index from themobile station, the first vector index corresponding to a first antennaprecoding vector in the selected subset; and to transmit first data tothe mobile station, using the first antenna precoding vector applied tothe transmitter antennas at the plurality of geographically separatedtransmitter sites.
 2. The arrangement of processing circuits of claim 1,wherein the one or more precoding selector circuits are configured toestimate path loss between a mobile station and each of the plurality ofgeographically separated transmitter sites by receiving channel statereports from the mobile station and estimating the path losses based onthe channel state reports, wherein the channel state reportscharacterize downlink path loss, received downlink signal quality, orboth.
 3. The arrangement of processing circuits of claim 1, wherein theone or more precoding selector circuits are configured to estimate pathloss between a mobile station and each of the plurality ofgeographically separated transmitter sites by estimating uplink pathlosses, based on measurements of uplink signals from the mobile station,and estimating corresponding downlink path losses based on the estimateduplink path losses.
 4. The arrangement of processing circuits of claim1, wherein the one or more controller circuits are configured totransmit site information to the mobile station prior to receiving thefirst vector index from the mobile station, wherein the site informationidentifies the plurality of geographically separated transmitter sites.5. The arrangement of processing circuits of claim 1, wherein the one ormore controller circuits are configured to: receive a second vectorindex from the mobile station, the second vector index corresponding toa second antenna precoding vector in the selected subset; and transmitsecond data to the mobile station, using the second antenna precodingvector applied to the transmitter antennas at the plurality ofgeographically separated transmitter sites.
 6. A method for determiningtransmission parameters in a mobile communications network usingcoordinated transmissions from multiple transmitter sites, the methodcomprising: estimating path loss between a mobile station and each of aplurality of geographically separated transmitter sites, eachtransmitter site having at least one transmitter antenna; selecting oneof a plurality of pre-determined subsets of a pre-determined set ofantenna precoding vectors, based on the estimated path losses;transmitting a group index to the mobile station, the group indexidentifying the selected subset of antenna precoding vectors;subsequently receiving a first vector index from the mobile station, thefirst vector index corresponding to a first antenna precoding vector inthe selected subset; and transmitting first data to the mobile station,using the first antenna precoding vector applied to the transmitterantennas at the plurality of transmitter sites.
 7. The method of claim6, wherein estimating path loss between a mobile station and each of theplurality of geographically separated transmitter sites comprisesreceiving channel state reports from the mobile station and estimatingthe path losses based on the channel state reports, wherein the channelstate reports characterize downlink path loss, received downlink signalquality, or both.
 8. The method of claim 6, wherein estimating path lossbetween a mobile station and each of the plurality of geographicallyseparated transmitter sites comprises estimating uplink path losses,based on measurements of uplink signals from the mobile station, andestimating corresponding downlink path losses based on the estimateduplink path losses.
 9. The method of claim 6, further comprisingtransmitting site information to the mobile station prior to receivingthe first vector index from the mobile station, wherein the siteinformation identifies the plurality of geographically separatedtransmitter sites.
 10. The method of claim 6, further comprising:receiving a second vector index from the mobile station, the secondvector index corresponding to a second antenna precoding vector in theselected subset; and transmitting second data to the mobile station,using the second antenna precoding vector applied to the transmitterantennas at the plurality of geographically separated transmitter sites.11. A method in a mobile station for determining transmission parametersin a mobile communications network using coordinated transmissions frommultiple transmitter sites, the method comprising: receiving transmittersite information identifying a plurality of geographically separatedtransmitter sites, each transmitter site having at least one transmitterantenna; determining a group index, the group index identifying aselected one of a plurality of pre-determined subsets of apre-determined set of antenna precoding vectors, wherein the determiningstep includes estimating path loss between the mobile station and eachof the plurality of geographically separated transmitter sites,selecting one of a plurality of pre-determined subsets of apre-determined set of antenna precoding vectors, based on the estimatedpath losses, and identifying the group index corresponding to theselected subset; transmitting the group index to the mobilecommunications network; evaluating channel conditions between thetransmitter antennas and the mobile station; selecting a first antennaprecoding vector from the selected subset, based on the channelconditions; and transmitting a first vector index to the mobilecommunications network, the first vector index identifying the firstantenna precoding vector, given the selected subset.
 12. The method ofclaim 11, further comprising processing signals subsequently receivedfrom the plurality of geographically separated transmitter sites usingthe first antenna precoding vector.
 13. The method of claim 11, furthercomprising: re-evaluating channel conditions between the transmitterantennas and the mobile station; selecting a second antenna precodingvector from the selected subset, based on the re-evaluated channelconditions; and transmitting a second vector index to the mobilecommunications network, the second vector index identifying the secondantenna precoding vector, given the selected subset.
 14. A mobilestation for use in a mobile communications network using coordinatedtransmissions from multiple transmitter sites, the mobile stationcomprising: one or more memory circuits storing a set of antennaprecoding vectors, wherein each antenna precoding vector is indexed by agroup index and by a vector index; a receiver circuit configured toreceive transmitter site information identifying a plurality ofgeographically separated transmitter sites, each transmitter site havingat least one transmitter antenna; a transmitter circuit; and one or morebaseband processing circuits configured to determine a group indexidentifying a selected one of a plurality of pre-determined subsets ofthe set of antenna precoding vectors; evaluate channel conditionsbetween the transmitter antennas and the mobile station; select a firstantenna precoding vector from the selected subset of antenna precodingvectors, based on the channel conditions; and transmit a first vectorindex to the mobile communications network via the transmitter circuit,the first vector index identifying the first antenna precoding vector,given the selected subset; wherein the one or more baseband processingcircuits are configured to determine the group index by: estimating pathloss between the mobile station and each of the plurality ofgeographically separated transmitter sites; selecting one of a pluralityof pre-determined subsets of a pre-determined set of antenna precodingvectors, based on the estimated path losses; and identifying the groupindex corresponding to the selected subset.
 15. The mobile station ofclaim 14, wherein the one or more baseband processing circuits arefurther configured to transmit the group index to the mobilecommunications network, via the transmitter circuit.
 16. The mobilestation of claim 14, wherein the receiver circuit is configured to usethe first antenna precoding vector to process signals subsequentlyreceived from the plurality of geographically separated transmittersites.
 17. The mobile station of claim 14, wherein the one or morebaseband processing circuits are further configured to: re-evaluatechannel conditions between the transmitter antennas and the mobilestation; select a second antenna precoding vector from the selectedsubset, based on the re-evaluated channel conditions; and transmit asecond vector index to the mobile communications network via thetransmitter circuit, the second vector index identifying the secondantenna precoding vector, given the selected subset.